Conventionally, there has been known an internal combustion engine in which blow-by of intake air (fresh air) occurs, as disclosed in Japanese Patent Laid-Open No. 2006-194112, for example. An internal combustion engine of this kind includes a turbocharger, and is provided with a predetermined amount of valve overlap amount under a high intake pressure situation during an operation of the turbocharger, whereby a part of intake air blows to an exhaust port. When the blow-by occurs, a cylinder filled air amount becomes smaller than an intake air amount predicted from an air flow meter.
Blow-by of fresh air like this is also called “scavenge”, and “scavenge control” that utilizes this is sometimes carried out. Scavenge control is to flow intake air out into an exhaust passage together with the combustion gas in a cylinder by using an overlap time period in which both an intake valve and an exhaust valve open, and has an effect of scavenging a cylinder residual gas. Hereinafter, an operation region where scavenge occurs will be also referred to as “a scavenge establishment region”.
In order to perform air-fuel ratio control precisely during scavenge control, the cylinder filled air amount needs to be obtained precisely after the blow-by amount of fresh air is taken into consideration. With an output value from the cylinder pressure sensor as a basis, an actual cylinder filled air amount can be calculated precisely. By using this point, the prior art according to the above described publication calculates the cylinder filled air amount based on the output value of the cylinder pressure sensor when blow-by of fresh air is recognized. Thereby, a suitable fuel injection amount is calculated from a control target air-fuel ratio with respect to the cylinder filled air amount, and the cylinder air-fuel ratio can be made to correspond to the control target air-fuel ratio. As a result, air-fuel ratio controllability can be kept.